My client wants to drop my flat retainer and pay me per sale instead

Three months into working with this client, things are going fine. Posts are consistent, engagement is up, and they are happy enough that they renewed once already.

Last week they floated a new idea. Instead of the flat monthly retainer, they want to pay me per sale that comes from social. Their thinking is simple: if I believe the work brings in business, I should be fine getting paid when it does.

I get the logic. I am just not sure the maths can ever be fair, for either of us.

Most of their sales do not happen through a neat tracked link. People see posts for weeks, then walk into the store, or DM a question and buy later, or get sent a post by a friend. We have no tracking links set up, no agreed window for how long after seeing a post a sale still counts, and no shared view of their sales numbers at all. Right now a per sale deal would mostly be both of us guessing, then quietly resenting the guess.

I did not say no outright. I said I would consider a test if we set it up properly first: a base fee that still covers the core work, one tracked link we both trust, a 14 day window we agree in writing, and one review at the end of the month where we both look at the same numbers. If any of that is missing, I would rather stay flat and keep the relationship simple.

They are thinking it over. I honestly do not know if the test is a good idea or just a slower way to end up arguing about attribution.

Has anyone here actually made performance pay work as a freelancer? What did you track, and what would you never agree to again?